A dam few Missoulians knew existed soon won’t.
If you didn’t grow up in the upper Rattlesnake like Arlin Grimes did, you may never have noticed the 3-million gallon reservoir hidden in the trees west of Larch Avenue. You wouldn’t have ridden bikes across the wooden bridge over the spillway, or swam and fished for cutthroat and rainbow trout in the pool below. You probably wouldn’t have chosen a career in river restoration, as Grimes did, and then jump at a chance to tear the old dam out and restore Rattlesnake Creek to free-flowing status.
“This project’s been on my radar for a few years, because of my connection to the creek here,” Grimes said on Monday as his Aqua Terra Restoration workers staged earth-movers at the site. “My inner child would like to see the fish moving through here again.”
Back in 1904, workers hand-piled boulders to make the initial earthen structure that became Rattlesnake Creek Dam and provided the city’s water supply. They eventually raised a mound about half as big as a football field retaining the reservoir, with a spillway that had a chlorination station and a small powerhouse. Montana Power Co. and then Mountain Water Co. sent drinking water down a 30-inch pipe for decades until a giardia scare in 1983 prompted the utilities to switch to wells drilled into the Missoula Valley aquifer. New security rules kinked the unofficial access kids like Grimes used to enjoy, and the dam literally faded from view behind the trees.
By the time the city government acquired Mountain Water in 2017, Rattlesnake Creek Dam did little more than pose a safety threat and block fish passage to the upper end of the Rattlesnake Valley. So the Missoula Water Division, city Parks and Recreation Department, Trout Unlimited and Montana Department of Fish, Wildlife & Parks teamed up to remove it and restore the old floodplain. More than 20 businesses, conservation groups and other entities supported the effort.
Missoula-based Waste Less Works deconstructed and removed a caretaker’s cabin and other structures on the 40-acre parcel. A Lolo National Forest project renovating the Lee Creek Campground along U.S. Highway 12 sent all of its cut trees and rootballs to the site for use rebuilding the riparian area. Trout Unlimited project manager Rob Roberts said that contribution cut a materials cost from about $80,000 to about $15,000.
Motorists on Rattlesnake Drive and bicyclists on the parallel bike path may hear but won’t see much of the reconstruction. That’s because Aqua Terra plans to use the 14,000 cubic yards of debris in the earthen dam to fill in the reservoir, essentially spreading the hump into a smooth creek grade. The concrete reservoir wall and other structures will be buried on site.
After the reservoir gets drained and dried out, workers will cut a bypass channel for the creek to run around the old concrete spillway. They will remove that barrier and add a new, curved channel for the creek, which will absorb much of the meltwater energy released every spring. Rattlesnake Creek only pushes about 50 cubic feet of water per second during the hot summers, but that can jump to 1,000 cfs during spring runoff. Because so much of the lower creek has been armored by residential development, the only place for that water to spread out has been a few places in Greenough Park that frequently suffer severe erosion. Roberts said the new floodplain should considerably reduce that hazard.
“This dam was never meant for flood control,” Roberts said. “In an average spring runoff, that reservoir would fill in about 20 minutes, and then the creek would flow right over the dam. When we’re done, it will have a five-acre floodplain that hopefully will absorb a lot of that energy. And we’ve added 10 miles of migration territory for the fish. We’re going to restructure this place to the point that in five years, nobody ever realizes there were manmade structures here.”